(1) Field of the Invention
The present invention relates to a thermographic dye transfer image-recording material and more particularly to one capable of providing images having enhanced image density.
(2) Description of the Related Art
It is well known that various cleavage reactions are assisted by silver ions including reactions involving cleavage of a compound into one or more fragments.
U.S. Pat. No. 3,719,489 discloses silver ion assisted cleavage reactions useful in photographic systems. As disclosed therein, photographically inert compounds are capable of undergoing cleavage in the presence of silver ions made available imagewise during processing of a silver halide emulsion to liberate a reagent, such as, a photographically active reagent or a dye in an imagewise distribution corresponding to that of said silver ions. In one embodiment disclosed therein, color images are produced by using as the photographically inert compounds, color providing compounds which are substantially non-diffusible in the photographic processing composition but capable of undergoing cleavage in the presence of the imagewise distribution of silver ions and/or soluble silver complex made available in the undeveloped and partially developed areas of a silver halide emulsion as a function of development to liberate a more mobile and diffusible color-providing moiety in an imagewise distribution corresponding to the imagewise distribution of said ions and/or said complex. The subsequent formation of a color image is the result of the differential in diffusibility between the parent compound and liberated color-providing moiety whereby the imagewise distribution of the more diffusible color-providing moiety released in the undeveloped and partially developed areas is free to transfer.
Color-providing compounds useful in the above process form the subject matter of U.S. Pat. No. 4,098,783, a continuation in part of said U.S. Pat. No. 3,719,489. The color-providing compounds disclosed therein may comprise one or more dye radicals and one or more 1,3-sulfur-nitrogen moieties. For example, they may comprise one complete dye or dye intermediate and one cyclic 1,3-sulfur-nitrogen moiety. Alternatively, the color-providing compounds may comprise two or more cyclic moieties for each dye radical or dye intermediate and vice versa. Particularly useful dye-providing compounds disclosed therein comprise a dye containing from 1 to 4 and preferably 1 or 2 cyclic 1,3-sulfur-nitrogen groups and may be represented by the formula EQU D--[(L).sub.m-1 --Y].sub.n (A)
wherein D represents a dye radical, i.e., the radical of an organic dye possessing at least one carbon atom, L is a divalent organic linking group containing at least one carbon atom, m is a positive integer 1 or 2, n is a positive integer from 1 to 4, and Y is a cyclic 1,3-sulfur-nitrogen group.
Thermally developable black and white as well as color photosensitive materials, which are imaged by light exposure and developed by heating, are well known. Among the systems designed to give color images are those wherein a diffusible dye is released as a result of the heat development of an organic silver salt and transferred to the image-receiving layer, whereby a color image is obtained.
Japanese Kokai 59-180548 having a Laid-Open date of Oct. 13, 1984 discloses a heat-developable silver halide photosensitive imaging system wherein the dye-providing material contains a heterocyclic ring containing a nitrogen atom and a sulfur or selenium atom which heterocyclic ring is subject to cleavage in the presence of silver ions to release a diffusible dye. An example of a suitable dye-providing material is a thiazolidine dye such as disclosed in the aforementioned U.S. Pat. No. 4,098,783. The process involves imagewise exposing the photosensitive system to light and subsequently or simultaneously heating the photosensitive system, in the presence of a base or base precursor, under a substantially water-free condition whereby an oxidation-reduction reaction between the exposed photosensitive silver halide and a reducing agent occurs. In the exposed areas a negative silver image is formed. In the unexposed areas, the silver ion, present in inverse proportion to the silver image, causes the heterocyclic ring of the dye-providing material to be cleaved releasing a diffusible dye. The diffusible dye is then transferred to an image-receiving layer whereby a positive dye image is formed.
A heat-developable photosensitive system useful in terms of thermal development of the silver halide latent image is one which comprises a support carrying a photosensitive silver halide, a silver salt oxidizer, a thermal solvent, a reducing agent for the silver salt, a binder, preferably gelatin, a dye-providing material capable of releasing dye upon silver ion assisted cleavage, and on the same or a separate support, an image-receiving layer capable of receiving the released dye. However, the use of conventional silver salt oxidizing materials has caused problems in this thermal system. Some of the salts have inadequate solubility in the thermal solvent, e.g., silver salts of compounds having an imino group such as silver benzotriazole and silver imidazole, so that there is an insufficient amount of soluble silver ion and/or silver salt complex available to cleave the dye-providing material. Other silver salt oxidizing materials, while they may have adequate solubility in the thermal solvents, bind the silver too weakly, e.g. silver saints of fatty acids such as silver behenate and silver stearate, so that there is release of dye during coating.
These problems are also present in thermographic color transfer imaging systems comprising a support carrying a dye-providing material capable of releasing a diffusible dye upon silver ion assisted cleavage, a binder, a thermal solvent, a silver salt, and on the same or a separate support an image-receiving layer capable of receiving the diffusible dye. These thermographic systems are imaged and developed by heat exposure. The process involves imagewise heating the thermographic image-recording material causing dissolution of the silver ions in an imagewise manner corresponding to said imagewise heating where they are then available to cleave the dye-providing material to release a diffusible dye in an imagewise pattern corresponding to said imagewise heating. The diffusible dye then transfers to the image-receiving sheet to form a dye image therein.
One attempt at solving the silver ion solubility problem has been to add an auxiliary ligand to the photothermographic or thermographic system such as disclosed in the copending U.S. patent application of James R. Freedman, et al, Ser. No. 07/923,858, filed Jul. 31, 1992, and assigned to the same assignee as the present application, now abandoned in favor of U.S. Ser. No. 08/079,146 filed Jun. 17, 1993. The auxiliary ligand is soluble in the thermal solvent and forms a complex with the silver ions. Including the auxiliary ligand in the photothermographic and/or thermographic system gives higher image densities, better image discrimination and accelerated silver development when compared with the same system(s) without an auxiliary ligand.
U.S. Pat. No. 4,260,677, issued Apr. 7, 1981, describes black and white thermographic and photothermographic materials wherein an image in reduced silver is formed. The thermographic materials comprise at least one layer comprising a binder, a reducing agent, and at least one silver salt complexed by at least one coordinating compound (ligand) having a gross stability constant between 4.50 and 10.00 wherein at least 90% of all silver salt within the layer is in the form of a silver salt complex with said at least one coordinating compound. The silver salt complex may be preformed or it may be formed in situ. Preferred coordinating ligands are described as those compounds containing an imidazole group. An advantage of the complexed silver salts is stated to be ease of coatability, i.e., that the complexes may be coated as a solution rather than a dispersion since the complexes dissolve in most coating solvents.